1. Field of the Invention
The present invention relates to a color picture tube.
2. Description of the Related Art
Generally, as shown in FIG. 2, a color picture tube includes an envelope composed of a face panel 31 in a substantially rectangular shape and a funnel 32 integrally connected to the face panel 31. On an inner surface of the face panel 31, a phosphor screen 33 is formed in a substantially rectangular shape, which is composed of stripe-shaped or dot-shaped phosphor layers of three colors respectively emitting blue, green, and red light. A shadow mask 34 in a substantially rectangular shape with a number of apertures formed in a substantially rectangular region corresponding to a substantially rectangular effective display region of a screen is attached to an inner wall of the face panel 31. On the other hand, an electron gun 37 emitting three electron beams 36R, 36G, and 36B is placed in a neck 35 of the funnel 32. The three electron beams 36R, 36G, and 36B emitted from the electron gun 37 are deflected by a horizontal deflection magnetic field and a vertical deflection magnetic field generated by a deflection yoke 38 mounted on an outer side of the funnel 32. Then, the electron beams 36R, 36G, and 36B are selected by the shadow mask 34, and a part thereof passes through the apertures to scan the phosphor screen 33 in horizontal and vertical directions, thereby displaying a color image.
The inner surface shape of the face panel 31 of the color picture tube is determined considering the transmittance of glass, the outer surface shape of the face panel 31, the uniformity of brightness, the uniformity of color, visibility, the inner surface reflection of the face panel 31, deflection distortion, the curved surface of the shadow mask 34, and the like. Generally, as shown in FIG. 3, the inner surface of the face panel 31 has a concave shape in which the circumference thereof is displaced in a direction approaching the electron gun side with respect to a center P0 of the inner surface of the face panel 31 through which a tube axis passes (e.g., see JP 55(1980)-28269 A).
For convenience in the following description, the displacement amount in a direction parallel to the tube axis at each position of the inner surface of the face panel 31 with respect to the center P0 will be referred to as a “sinking amount”. Furthermore, an axis, which is orthogonal to the tube axis in a direction parallel to a short side of the face panel 31, will be referred to a short axis, and an intersection point between a surface including the short axis and the tube axis, and a circumferential edge of the effective display region of the face panel 31 will be referred to as a short axis direction end. Furthermore, an axis, which is orthogonal to the tube axis in a direction parallel to a long side of the face panel 31, will be referred to as a long axis, and an intersection point between a surface including the long axis and the tube axis and the circumferential edge of the effective display region of the face panel 31 will be referred to as a long axis direction end. Furthermore, an intersection point between a surface including a diagonal axis of the effective display region in a rectangular shape and the tube axis, and the circumferential edge of the effective display region of the face panel 31 will be referred to as a diagonal axis direction end.
Since the effective display region of the face panel 31 has a substantially rectangular shape, the respective distances from the center P0 of the inner surface of the face panel 31 to the short axis direction end, the long axis direction end, and the diagonal axis direction end are different from each other. In the case where the sinking amounts with respect to the center P0 at the short axis direction end, the long axis direction end, and the diagonal axis direction end are varied in accordance with the difference in distance (i.e., in the case where the sinking amount is set to be larger with distance from the center P0), each sinking amount of the inner surface of the face panel 31 along the short axis, the long axis, and the diagonal axis changes quadratically.
However, in the case where the sinking amounts with respect to the center P0 at the short axis direction end, the long axis direction end, and the diagonal axis direction end are set to be the same, in particular, the sinking amount of the inner surface of the face panel 31 along the diagonal axis does not change quadratically, and a change curve of the sinking amount has an inflection point in a region 51 in the vicinity of the diagonal axis direction end farthest from the center P0, as shown in FIG. 4.
Recently, in order to reduce the reflection of outside light on the inner surface of the face panel 31 to enhance contrast, tinted glass having a small transmittance with respect to visible light is used sometimes. In the face panel 31 using tinted glass (such a face panel will be referred to as a “tinted panel”), when the thickness of glass varies in the effective display region, the uniformity of brightness degrades remarkably. Thus, it is preferable to minimize the difference in sinking amount in the tinted panel. This makes it impossible to increase the sinking amount at the diagonal axis direction end farthest from the center P0, and consequently, the change curve of the sinking amount is likely to have an inflection point in the tinted panel, as shown in FIG. 4.
Furthermore, at present times, there is a tendency that the outer surface of the face panel 31 is flattened. It is relatively easy to reduce the difference in thickness of glass with respect to the center P0 in the effective display region in a conventional face panel having a convex curve on an outer surface, which makes it relatively easy to maintain the uniformity of brightness in the case of using tinted glass. However, in order to flatten the outer surface of the face panel 31 while keeping the thickness at each position in the effective display region to be the same as that of the face panel having a convex curve on an outer surface, it is necessary to reduce the sinking amount at the circumference with respect to the center P0 of the inner surface of the face panel. Consequently, in the face panel with the outer surface flattened, the change curve of a sinking amount is likely to have an inflection point, as shown in FIG. 4.
As shown in FIG. 4, in the case where the change curve of a sinking amount of the inner surface of the face panel 31 has an inflection point, the state of a film such as the phosphor screen 33 formed on the inner surface of the face panel 31 changes, compared with the case having no inflection point. This will be described below.
Generally, in a color picture tube, as means for forming a film on the inner surface of a face panel, an exposure and development system is used. According to this system, the following usually is performed. A film material is applied to an inner surface of a face panel, which is rotated to form a thin film over the entire surface, and exposed to light using a shadow mask as an exposure mask, followed by development.
When a coating film is formed by the exposure and development system, in the case where the change curve of a sinking amount of the inner surface of a face panel does not have an inflection point, a coating film with a thickness varied gradually from the center to the circumference is obtained. On the other hand, in the case where the change curve has an inflection point, a coating film is formed in which the thickness is varied irregularly after the inflection point, and generally is small irregularly on the circumferential side with respect to the inflection point.
Such an irregular variation in thickness can be corrected by changing the setting of an exposure system, such as adjusting an exposure amount. However, there is a limit to the correction, and in some cases, a phenomenon such as overexposure occurs due to the extremely small thinness of a film, degrading the screen quality remarkably
As a specific example, the case will be considered where a black matrix, which is a black non-light-emitting substance to be applied so as to mainly enhance a tube surface color in a color picture tube, is fixed. Generally, in order to fix the black matrix, the following processes are performed: coating of a resist film on the inner surface of a face panel, mask exposure via a shadow mask, development of the resist film, coating of a black matrix, and removal of a developed resist portion. In these processes, when an irregularly thin portion is present in the applied resist film, an exposure region becomes large in the thin portion, and the fixing amount of the black matrix decreases irregularly in that portion. Consequently, the size of a phosphor region (phosphor size) to be formed in a non-fixed portion of the black matrix increases.
Alternatively, in an excessively thin portion of the applied resist film, an overexposure phenomenon of a resist occurs, causing the burning of the resist. Irrespective of whether the resist is developed in that portion, it is not removed finally. Thus, the black matrix on the resist is not removed, either, and consequently the black matrix adheres to an undesired portion, which reduces a phosphor size.
Thus, the irregular variation in thickness, in particular the variation in which the thickness decreases irregularly, degrades screen quality, and further, causes a remarkable degradation in image quality.